This invention relates to an improved process and apparatus for liquid acid catalyzed continuous alkylation in which the acid inventory is reduced.
Alkylation is a reaction in which an alkyl group is added to an organic molecule Thus, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C2 to C5 olefin with isobutane in the presence of an acidic catalyst producing a so called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also its sensitivity to octane enhancing additives.
Petroleum refining processes have historically used both sulfuric acid and hydrofluoric acid (HF) as catalysts in commercial alkylation. The sulfuric acid alkylation reaction is particularly sensitive to temperature, with low temperatures being favored to minimize the side reaction of olefin polymerization. Acid strength in these liquid acid catalyzed alkylation processes is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. The hydrofluoric acid process is less temperature sensitive and the acid is easily recovered and purified.
U.S. Pat. No. 4,795,728 to Kocal teaches a hydrofluoric acid catalyzed alkylation process for producing motor fuel. The hydrofluoric acid catalyst complex contains from 0.5 to 5 weight percent of a cationic or anionic surfactant component enabling the process to be operated at an olefin:acid volumetric feed ratio of greater than 1.0 while maintaining acceptable alkylate quality.
For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C4 Olefins", 27 Ind. Eng. Chem. Res., p. 381-397 (1988).
HF alkylation is described in further detail in the Handbook of Petroleum Refining Processes, p. 3-28 (1986).
Generally, in modern acid alkylation longer residence times for the hydrocarbon/acid contact are preferred. However, longer residence times result in reduced reactor capacity as well as increased operating costs. For a discussion of residence time see Albright, "Modern Alkylation", Oil and Gas Journal, p. 83, (Nov. 12, 1990).
Lewis acid catalyzed alkylation processes are also currently used to produce high octane blending components. Examples of Lewis acids include BF.sub.3, AlCl.sub.3 and SbF.sub.5.
Liquid acid catalyzed continuous alkylation processes generally comprise a reactor, a settler where hydrocarbon droplets are separated from the acid and a heat exchanger where the heat generated by the exothermic reaction is removed. Each vessel requires a large liquid acid catalyst inventory.
Both sulfuric acid and HF alkylation share inherent drawbacks including environmental and safety concerns and acid consumption. While catalyst complexes comprising BF.sub.3 overcome some of the safety and environmental drawbacks of sulfuric acid and HF alkylation systems, the volume and quality of BF3 alkylates have not, in the past, proven comparable to that of sulfuric or HF alkylates. Currently HF catalyzed alkylation processes are under particular safety and environmental scrutiny, because of the toxic and corrosive nature of HF.
U.S. Pat. Nos. 4,938,935 and 4,938,936 describe the danger of HF leaks. Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape creating a vapor cloud that can be spread for some distance.
It is therefore an object of the present invention to provide a process and apparatus for reducing the liquid acid catalyst inventory in acid-continuous alkylation processes.
It is a further object to provide a process and apparatus for improving the safety of liquid acid catalyzed continuous alkylation.
It is a further object of the present invention to provide an apparatus for minimizing the risk of a sudden release of toxic material.